docs: def split(self, grouper):
          ''' Split the current SparseRunVariable into multiple columns.
  
          Args:
              grouper (iterable): list to groupby, where each unique value will
                  be taken as the name of the resulting column.
  
          Returns:
              A list of SparseRunVariables, one per unique value in the
              grouper.
          '''
          data = self.to_df(condition=True, entities=True)
          data = data.drop('condition', axis=1)
  
          subsets = []
          for i, (name, g) in enumerate(data.groupby(grouper)):
              name = '%s.%s' % (self.name, name)
              col = self.__class__(name=name, data=g, source=self.source,
                                   run_info=getattr(self, 'run_info', None))
              subsets.append(col)
          return subsets